Thickening agents are used for thickening of the most diverse liquids. Natural and synthetic oils, water, aqueous electrolyte solutions and aqueous solutions of organic substances are cited as examples.
An important field of application of thickening agents is the preparation of functional liquids such as, for example, hydraulic fluids and metalworking fluids, on which stiff requirements regarding viscosity, shear stability and temperature behavior are imposed.
Thickening agents based on polyether derivatives are already known. Thus, in U.S. Pat. No. 4,521,326, the disclosure of which is expressly incorporated herein by reference, thickening agents based on polyether derivatives are described which are synthesized by adding alkylene oxide, namely ethylene oxide and/or propylene oxide, to an alcohol with at least 10 carbon atoms, and then adding a long-chain 1,2-epoxide with a chain length of 10 to 32 carbon atoms to the polyalkylene glycol monoether obtained thereby.
Further thickening agents, also synthesized on the basis of polyethers, are described in West German Laid-open Application 3,630,319. In the process indicated therein, monohydric alcohols with 8 to 30 carbon atoms are alkoxylated with a mixture of ethylene oxide and propylene oxide, the molar ratio of ethylene oxide to propylene oxide in the mixture being approximately 30:70 to 90:10 and 20 to 200 moles of alkylene oxide being used per mole of alcohol, and the resulting polyether is reacted in a molar ratio of 1:0.7 to 1:0.25 with a diisocyanate. The disclosure of this West German Laid-open Application is expressly incorporated herein by reference.
With such thickening agents it is possible significantly to increase the viscosity of numerous liquids. The polyether derivative can be dissolved directly in the liquid to be thickened; however, it is also possible to prepare so-called stock solutions and then use these for thickening.
One disadvantage in this procedure is that during use of such thickening agents, especially if they exist in solid form, prolonged stirring is needed in order to make the thickener effective. Problems can occur in many cases if the thickener can be dissolved only with difficulty in the liquids to be thickened, e.g., if the temperature must be raised in the dissolution process or if special stirring devices are necessary.
These difficulties can be partly circumvented by using so-called stock solutions, wherein a thickening-agent solution of the highest possible concentration is prepared and this stock solution is used for thickening the liquid.
A disadvantage in working with these stock solutions is that frequently it is not possible to prepare stock solutions with high concentrations, since the viscosity of the stock solutions increases rapidly with increasing concentration, so that such stock solutions are difficult to handle.
U.S. Pat. No. 4,491,526 describes thickening agents that can contain various polyether derivatives and that must also contain at least one surface-active ethoxylated polyether. Besides, it is recommended that an entire series of other additives including ethoxylated phosphoric acid esters be added to these thickening agents.
As a result of the addition of the surface-active ethoxylated polyethers, these thickening agents have a tendency to foam. Furthermore, they are highly viscous, especially at relatively high concentrations of the polyether derivative and of the surfactant. Also, in order to be able to dissolve relatively large quantities of the polyether derivative in water, a considerable quantity of surface-active substance must be added.
Although numerous thickening agents based on polyethers are already known, a need still exists for improved thickening agents, especially for such which exhibit the said disadvantages not at all or only to a reduced extent.